Significance If learning contributes to the development of vocal perception in the squirrel monkey, then this will represent an important parallel between primate vocal development and human language development and may provide an important nonhuman primate model for studying language development in human children. Objectives We experimentally exposed subjects of four different age classes, under controlled conditions, to the chuck calls of genetically unrelated individuals of different social affinities. Using playback experimentation, we tested the hypothesis that subjects at different developmental stages respond preferentially to the chuck calls of socially close but unrelated companions when compared to those of other individuals either from within or outside of the social group. Results Results demonstrated that subadults behaviorally respond preferentially to the chuck calls of close associates but infants, juveniles, and adults do not. Juveniles behaviorally responded more strongly to the playback of chuck calls arising from individuals within their colony when compared to the playback of "silence." These results demonstrate that learning plays a role in chuck call recognition in squirrel monkeys and suggest that infants gradually learn to recognize social companions vocally within their groups. Future Directions Further research is needed to determine when in development vocal recognition emerges and whether a sex difference is found in the chuck recognition system in squirrel monkeys by examining both behavioral and vocal responses from infants, juveniles, and subadults to chuck call playback from genetically unrelated adults. KEY WORDS vocal learning, language development, evolution of language, bioacoustic analysis FUNDING NIH NRSA Fellowship 1-F32-HD07946